PROFILES IN Garbage: Yard Waste

Yard waste includes grass, leaves and tree and brush trimmings. Grass is the biggest yard waste component by weight, while leaves are the biggest component by volume. Local yard waste generation varies dramatically based on climate, yard size and the percentage of the population in single family housing.

Yard waste can be the largest component of municipal solid waste (MSW) during the summer and fall, with grass predominanting in the summer and leaves predominating in the fall.

Yard waste also is the largest single component of generated MSW by weight and one of the largest components of landfilled MSW by volume.

* Large scale leaf composting is a form of volume reduction resulting in the loss of 40 percent to 75 percent of the original volume and 50 percent of the original weight.

The Composting Process: Composting is the controlled decomposition of organic matter by microorganisms into a humus-like product. Techniques such as windrows, static piles and in-vessel systems generate energy and heat, and destroy weeds, plants and human pathogens. Water and carbon dioxide dissipate into the atmosphere during this process.

To maintain aerobic conditions, yard waste usually is turned to provide oxygen for the composting organisms. Temperature control (132 degrees to 140 degrees Fahrenheit), moisture content (40 percent to 60 percent) and an adequate carbon-to-nitrogen ratio are required.

Insufficient aeration or an improper car-bon-to-nitrogen ratio can cause odors. Additionally, an improper operation can allow a fungus, aspergillus fumigatus, to grow on compost piles and cause health problems.

Compost can be produced in three to 18 months, depending on the process and amount of yard waste used.

Composting Markets: Yard waste compost is not a fertilizer. It is a useful soil conditioner that improves texture, air circulation and drainage. Compost can moderate soil temperature, enhance nutrient and water-holding capacity, decrease erosion, inhibit weed growth and suppress some plant pathogens.

High-quality compost can be marketed as a soil amendment and as mulch to landscapers, farmers, nursery owners and the general public. Compost can be used for highway embankments, parks and school grounds in place of topsoil and mulch. Farm soil restoration is a potential high-growth market, and compost also can be used as a daily cover for landfills.

End-Market Specifications: Each end market has its own specifications, with limits on moisture and other potential contaminants. Generally, non-organic materials (glass metals, plastic bags, etc.) must be kept separate from yard waste. Tests show little heavy metal contamination of yard waste.

Composting Cost and Value: A Composting Council, Bethesda, Md., study of yard waste composting facilities showed an average processing cost of $25 per ton, with a median of $16 per ton and range of $8 per ton to $72 per ton.